Heat-transfer tubes which are incorporated and used in a steam generator in nuclear power facilities, and heat-transfer tubes which are incorporated in a heat exchanger in a feed water heater etc. of various equipments are produced as a longer-length tube having, for example, a small outer diameter of not more than 40 mm and a length of not less than 15 m. Such longer-length, small-diameter tubes are generally produced by preparing, as a starting material, a seamless tube which is produced in a hot working method using a Ugine Sejournet tube-making facility, and subjecting it to a bright heat treatment in a reducing atmosphere and thereafter a cold drawing.
It is common practice in cold drawing to form chemically treated lubricating films on the inner and outer surfaces of a blank tube after a heat treatment. However, when a chemically treated lubricating film is formed on a longer-length, small-diameter tube, care must be taken to ensure that the blank tube is sufficiently treated for the entire inner surface at the time of chemical treatment. Such a treatment requires huge man-hours, and chemicals used in the treatment are relatively expensive, thus resulting in incurring high operation costs. Further, in the case of Ni-based alloys (Inconel type alloys) used for SG tubes in nuclear power facilities, a problem arises that a chemically treated lubricating film is not likely to be formed on such a material.
For that reason, in recent years, an oil-lubricated drawing in which an oil lubricating film is formed on the inner and outer surfaces of the blank tube has been practiced. In this drawing method, chemicals are less expensive and the processing thereafter is relatively easier compared to the case of forming a chemically treated lubricating film.
Further, as an improved method categorized in the oil-lubricated drawing method in which an oil lubricating film is formed on the surface of the blank tube, a high-pressure lubrication drawing method (a high-pressure draw method) has been developed. The method stabilizes the drawing and achieves significant effects in improving the quality of the drawn tube by constantly supplying a high-pressure lubricating oil between the blank tube and a tool. This is a method of processing a tube in which a blank tube is placed inside a high-pressure container which is filled with a lubricating oil, and the blank tube is pulled outside the high-pressure container during which drawing is performed while feeding high-pressure lubricating oil.
FIG. 1 is a diagram to illustrate a high-pressure lubrication drawing method, in which drawing is performed while feeding high-pressure lubricating oil. In FIG. 1, when the drawing of a blank tube 3 is performed using a plug 1 and a die 2, a cylindrical container 4 of which one end is closed and an open end has a telescopic structure 4a is swingably provided with the closed end side being a fulcrum such that the open end side can be changed in orientation between a drawing pass line and a blank tube insertion line.
Penetratingly disposed in the container 4 is a plug supporting rod 5 for retaining the plug 1 such that the plug 1 is placed in the die 2 which is securedly disposed on the drawing pass line.
The blank tube 3 which has been subjected to a bright heat treatment is loaded in the container 4, and is set in a state where a pointed portion of the blank tube 3 is passed through an annular space formed by the die 2 and the plug 1 as shown in FIG. 1.
In this state, a high-pressure lubricating oil is fed to fill the container 4 by a pump P, the blank tube 3 is drawn through the annular space to outside the container 4 to be formed into a drawn tube having predetermined dimensions. Throughout the process of this drawing, the inner and outer surfaces of the blank tube 3 are continuously fed with the high-pressure lubricating oil which is fed to fill the container 4.
At this moment, the pressure tightness between the open end and the die 2 in the container 4 is automatically maintained as the result of that the telescopic structure 4a provided on the open end side in the container 4 is pressed in the left hand direction in the drawing by the high-pressure lubricating oil, and thereby the front end thereof is brought into pressure contact with the entrance side surface of the die 2. Further, the pressure tightness between the plug 1 and the die 2 is maintained by the blank tube 3 which is being drawn.
With this high-pressure lubricated drawing method, lubricating oil is fed to fill between the blank tube and the tool by high pressure, and therefore oil is not likely to run out during drawing, making it possible to substantially prevent scoring which is likely to occur in a typical oil-lubricated drawing. However, even when the high-pressure lubricated drawing method is adopted, there may be cases where scoring occurs locally and where chattering vibration occurs.
If scoring occurs during drawing, the surface quality of tube product degrades, leading to a decline in yield. Moreover, if chattering vibration occurs, the inner diameter of the tube product fluctuates, although by a very small amount, along the longitudinal direction. When such a tube product is used as an SG tube in nuclear power facilities, a rigorous inspection standard is set in eddy-current examination with inner coil method, and since the inner diameter fluctuation along the longitudinal direction generates a background noises, it will remarkably reduce the S/N ratio (S: signal from a flaw, N: noise) of the tube, being sentenced as nonconformance.
Regarding the prevention of occurrence of scoring and chattering vibration, various proposals have been made until now. For example, Patent Literature 1 describes a drawing method in which to prevent chattering vibration which occurs in a metal to be processed, a bright heat treatment in a hydrogen atmosphere having a dew point of −50° C. or less is applied to the workpiece before drawing. It is stated that suppressing the generation of chromium oxide (Cr2O3), alumina oxide (Al2O3) and the like during heat treatment makes it possible to restrain the fluctuation of the friction coefficient between the blank tube and the tool during drawing, thus preventing chattering vibration.
Patent Literature 2 describes a plug to be used for drawing work of a workpiece, which has been subjected to a lubrication treatment (a blank tube having been subjected to a bright heat treatment), wherein the surface roughness Rmax of an area that comes into contact with the workpiece is 0.4 to 2.0 μm, and a method for producing a drawn steel tube by using the plug. It is stated that since metal oxide is trapped in minute concave portions which are present on the plug surface, and fine powder of metal oxide that flows out thereof tends to cut the lubricating oil film, thereby increasing friction resistance and causing chattering vibration, the occurrence of chattering vibration can be prevented by appropriately adjusting the surface roughness of the plug to secure oil pits having sufficient capacity to harbor lubricating oil.
The technologies described in Patent Literatures 1 and 2 are respectively an effective method for preventing the occurrence of chattering vibration. However, the prior art is not necessarily perfect, when applied alone, and occasionally local scoring and chattering vibration happen to occur depending on the state of the inner surface of blank tube which is the workpiece, the state of formation of oil lubricating film, and the conditions of drawing, etc.